1. Field of the Invention
The present invention pertains to radiation measurement instruments and, more particularly, to instruments and a method for monitoring the biological effects of radiation in the ultraviolet-B band.
2. Discussion of the Prior Art
Solar radiation consists of ultraviolet, visible and infrared light. Ultraviolet (UV) light has the shortest wavelengths, ranging from 200 to 400 nanometers (nm) and is further subdivided into UV-C with wavelengths from 200 to 290 nm, UV-B ranging from 290 to about 320 nm and UV-A in the 320 to 400 nm range. UV-B radiation has been the subject of recent scientific and medical interest because of its potential carcinogenic effects on humans and impact on ocean and plant life. In addition, publicized damage to the stratospheric ozone layer is thought to be dangerously increasing the amount of UV-B radiation incident on the earth's surface.
Within the past decade, the NASA satellite instrument TOMS (Total Ozone Mapping Spectrometer) has been recording large decreases in the stratospheric ozone over Antarctica and to a lesser extent over the northern polar regions. Simultaneous ground based measurements of UV-B in Antarctica consistently show a corresponding inverse correlation as a function of stratospheric ozone concentrations. It is becoming increasingly important to accurately measure and monitor UV-B and the accompanying biological effects, and recently a number of workshops have been conducted to address the problems associated with obtaining such information.
One accepted measure relating incident UV-B energy to a particular biological effect, i.e., sunburn, is the McKinlay-Diffey (M-D) erythema action spectrum. The M-D spectrum defines the onset of sunburn as a function of the radiant intensity, or surface irradiance, and the wavelength of the incident radiation. By multiplying the surface irradiance in each increment of the 290-320 nm UV-B spectrum by the corresponding M-D irradiance and summing the values to approximate a weighted integral, a single value estimator of the sunburning potential of an environment can be derived. The values of the M-D relationship are known and fixed but the incident UV-B radiation at any point and time must be measured. The Robertson-Berger meter (RB meter) was designed in the early 1970's as a low cost means of generally correlating incident UV-B radiation with the erythema relationship. The RB meter uses four levels of filtration or wavelength conversion to obtain a signal that is roughly proportional to the UV-B radiation weighted by the response function of the instrument. Although not exact, there is a similarity between the RB meter response and the accepted erythema (sunburn) response of average Caucasian skin. Solar radiation enters the instrument from the hemisphere with a quasi-Lambertian response (i.e., a response that varies as the cosine of the angle of incidence with respect to a plane perpendicular to the plane of the input aperture) where it is integrated by a diffuser. This diffused radiation then passes through a Schott UG11 glass filter transmitting in two bands: 250 to 400 nm and 700 to 850 nm, respectively (the long wavelength transmission is small compared to the UV). This radiation then passes to a MgWO.sub.4 phosphor screen excited only by the UV and produces an output in a wavelength band from 350 to 650 nm. The light then passes through a Corning 4010 glass filter transmitting only between 500 and 600 nm. Finally, the radiation falls on the cathode of a 1P39 vacuum phototube sensitive to 300 to 650 nm. The result is a solar blind detector system that responds to UV-B.
A network of RB meters was set up to monitor sunburning radiation around the globe but a review of the RB meter output at eight field sites in the continental U.S. showed a surprising trend of decreasing UV-B over the twelve years considered (1974-1985) despite increasing evidence of ozone layer weakening. This decrease is assumed to reflect an increase in cloudiness and an increase in tropospheric ozone concentrations in the urban sites selected for inclusion in the RB network, thereby masking the effects of the decrease in ozone attributed to chlorinated fluorocarbons (CFC).
The difficulty is that cloud cover, one of the two major atmospheric influences on incident UV-B, affects the spectrum of solar radiation uniformly from UV through visible and into the infra-red (IR) band; but the second major influence, the ozone level, attenuates the shorter wavelengths in the UV-B spectrum much more than those higher in the range.
The RB meter, developed before the recent interest in the depletion of the ozone layer, responds to total irradiance across the UV-B wavelength spectrum and operates on the assumption that the energy in each wavelength increment follows a characteristic predictable distribution. This was a reasonable approach so long as the ozone filter of UV-B radiation remained essentially constant, but is insufficient when the varying effects of ozone action is the focus of inquiry.
Measuring solar UV with a single detector, even a solar-blind device, does not satisfy the need for accurate determination of the biological effects of incident radiation in the face of changing ozone filter. Both the quality and quantity of biological effects are incorrectly inferred from the output of a single detector. The single value for total UV-B irradiance produced by such a detector can correspond to an infinite number of different energy-frequency distributions, each potentially having different biological implications based on the independent and uncorrelated influences of cloud cover and ozone absorption. In addition, the phosphor convertor screen, the UV filter, the visible filter and the photocathode of the phototube of the existing RB meter design all have potential for deterioration and drifting, thereby degrading the instrument response.
The alternative approach to accurately assessing surface UV-B irradiance as a function of wavelength has required bulky, expensive spectrographic equipment and expert operators.